(1) Field of the Invention
The present invention relates to an optical waveguide filter to be utilized for various optical communication devices, in particular, to an optical waveguide filter for selecting light of desired wavelength by utilizing polarization mode conversion.
(2) Related Art
FIG. 20 is a plan view showing a constitutional example of a conventional optical waveguide filter utilizing polarization mode conversion. In the conventional optical waveguide filter as shown in FIG. 20, light input to an input port PIN is input to a polarization beam splitter (PBS) 11 via a waveguide 1A, to be polarization split into light in TE mode and light in TM mode. The light in TE mode and the light in TM mode pass through a waveguide 2B and a waveguide 2A, respectively, to be input to a polarization mode converter 12. At the polarization mode converter 12, only the light of desired wavelength is subjected to the polarization mode conversion, due to an effect of, for example, acousto-optics effect, electro-optics effect, photoelastic effect or the like. Then, the light that has passed through the polarization mode converter 12 is sent to a succeeding stage polarization beam splitter 13 and is split for each mode. The wavelength light that has been subjected to the mode conversion at the polarization mode converter 12 passes through a waveguide 3B and is emitted as a selected light from an output port POUT. The wavelength light that has not been subjected to the mode conversion passes through a waveguide 3A and is emitted as a non-selected light from an output port POUTxe2x80x2. Here, in FIG. 20, mode components corresponding to selected light are denoted in capital letters xe2x80x9cTE, TMxe2x80x9d, and mode components corresponding to non-selected light are denoted in small letters xe2x80x9cte, tmxe2x80x9d. The optical waveguide filter of such a constitution is utilized as a wavelength filter of polarization independency in various optical communication devices.
According to such a conventional optical waveguide filter as described in the above, however, there causes a problem in that it is difficult to obtain a large extinction ratio between the light of selected wavelength and the light of wavelength other than the selected wavelength. That is, since it is difficult to obtain a large splitting rate of the polarization beam splitter with respect to each of the light in TE mode and the light in TM mode, as shown by parenthesized mode components xe2x80x9c(TE), (TM)xe2x80x9d in FIG. 20, a non-split component that could not be polarization split into each mode at the polarization beam splitter 11, is output from each output end of the polarization beam splitter 11. Further, since the light in each mode including the non-split component passes through the polarization mode converter 12 to be sent to the polarization beam splitter 13, the non-split component is output from each output end of the polarization beam splitter 13. Therefore, when the splitting rate of each of the polarization beam splitters 11 and 13 is small, the non-split light of wavelength other than the selected wavelength is emitted from the output port POUT for the selected light, resulting in deterioration of extinction ratio characteristics of selected light and of non-selected light.
The present invention has been achieved in view of the abovementioned problem and has an object to provide an optical waveguide filter that has realized satisfactory extinction ratio characteristics by reliably selecting light of desired wavelength by utilizing polarization mode conversion. Also, the present invention has an object to provide a technique for realizing the low-loss and miniaturization of optical waveguide filter utilizing polarization mode conversion.
In order to achieve the above objects, one aspect of an optical waveguide filter according to the present invention comprising: an input side polarization splitting section that splits light input to an input waveguide into two polarization components, and outputs one component of the two polarization components to a first waveguide and the other component to a second waveguide; a mode converting section that performs polarization mode conversion on light corresponding to selected wavelength among respective lights being propagated through the first and second waveguides; and an output side polarization splitting section that splits the respective lights being propagated through the first and second waveguides after having passed through the mode converting section, in accordance with polarization states thereof, to output the light of the selected wavelength to an output waveguide, comprises a first polarization splitting section disposed on the first waveguide positioned between the input side polarization splitting section and the output side polarization splitting section, and a second polarization splitting section disposed on the second waveguide positioned between the input side polarization splitting section and the output side polarization splitting section, wherein the respective lights being propagated through the first and second waveguides are split at the first and second polarization splitting sections in accordance with polarization states thereof, to eliminate non-split components generated corresponding to a splitting rate of the input side polarization splitting section.
According to the optical waveguide filter of such a constitution, the light input to the input waveguide is polarization split at the input side polarization splitting section and the split lights are sent to the first and second waveguides, respectively. Among the lights being propagated through the respective waveguides, the light corresponding to the selected wavelength is subjected to TE-TM mode conversion or TM-TE mode conversion at the mode converting section, while the light of other wavelength is sent to the output side polarization splitting section just as it is. At this time, the lights being propagated through the respective waveguides include, respectively, non-split components that could not be polarization split at the input polarization splitting section. However, since the propagated lights pass through the first and second polarization splitting sections, respectively, the respective non-split components are eliminated. Then, the lights propagated through the first and second waveguides, respectively, are polarization split at the output side polarization splitting section, so that the light of selected wavelength is output to the output waveguide. Thus, it becomes possible to obtain selected light of desired wavelength in which an influence of non-split components generated corresponding to the splitting rate of the input polarization splitting section is reduced, hence enabling the improvement of extinction ratio characteristics of selected light and of non-selected light.
Moreover, another aspect of the optical waveguide filter according to the present invention comprising the input side polarization splitting section, mode converting section and output side polarization splitting section as described above, comprises: a first polarization splitting section having a crossing waveguide shape that is disposed on a first waveguide positioned between the input side polarization splitting section and the output side polarization splitting section, and is capable to linearly locate the waveguide positioned between the input side polarization splitting section and also to linearly locate the waveguide positioned between the output side polarization splitting section; and a second polarization splitting section having a crossing waveguide shape that is disposed on a second waveguide positioned between the input side polarization splitting section and the output side polarization splitting section, and is capable to linearly locate the waveguide positioned between the input side polarization splitting section and also to linearly locate the waveguide positioned between the output side polarization splitting section.
According to the optical waveguide filter of such a constitution, the first polarization splitting section and the second polarization splitting section, each having a required crossing waveguide shape, are disposed on the first waveguide and the second waveguide, respectively, so that the input side polarization splitting section and the output side polarization splitting section are connected by each zone of the linear waveguide without curve. Thus, it becomes possible to reduce a loss within the optical filter due to a curve loss of waveguide or the like, and also to achieve the miniaturization of optical filter since the waveguide layout is simplified.
Other objects, features and advantages of this invention will become apparent in the following description of embodiments in relation to the attached drawings.